The present invention pertains generally to piezoelectric ceramic compositions and in particular to lead zirconate titanate (PZT) ceramics.
Lead zirconate titanate (PZT) has exceptional dielectric and piezoelectric properties and accordingly is used frequently in electromechanical transducers for producing, sensing, and measuring sound, pressure, vibration, and shock. This ceramic is a polycrystalline solid state solution of lead zirconate (PbZrO.sub.3) and lead titanate (PbTiO.sub.3). Typically, aggregates of the ceramic are shaped, heat treated, and then poled to form a transducer.
A serious problem associated with PZT transducers is the deterioration in the dielectric constant (.epsilon..sub.o.sup.33) and piezoelectric response (d.sub.33) with time, often being referred to as aging. Due to this problem, piezoelectric systems require frequent maintenance and replacement which is expensive and difficult in deep-sea environments.
The PZT composition, like any other ceramic composition, has been frequently modified with the addition of a small amount of one or more other materials, for example oxides or halides. The modifications have been made to improve physical properties, e.g., density, and electrical properties, e.g., the electromechanical coupling factor.
In Kulcsar, U.S. Pat. No. 2,911,370, at least one oxide of the group consisting of niobium, tantalum, and rare earth elements except cerium in an amount from about 0.1 to 5 weight percent on an oxide basis and a carbonate or other compound of calcium and/or strontium in an amount sufficient to impart up to 20 atomic percent of these two alkaline earth elements are added to a PZT composition to improve aging. Kulcsar also discusses the difficulty of improving one property without seriously impairing one or more properties.
Improved aging properties, along with improved combinations of mechanical quality factor, Q, and mechanical coupling, were obtained by the modified PZT ceramics of U.S. Pat. No. 3,179,594 by Kulcsar et al. The general range of PbZrO.sub.3 :PbTiO.sub.2 mol ratios is from 60:40 to 35:65 wherein from 1 to 10 atomic percent of lead is replaced with magnesium. The lead can be further replaced with 1 to 10 atomic percent of one or more alkaline earth elements selected from the class consisting of barium, strontium, and calcium. The substituents are added as oxides or carbonates.
Oxides and carbonates have been added to PZT ceramics to improve the mechanical quality factor (Q), mechanical coupling, and density. Lead in a PZT ceramic composition is replaced by lanthanum in an amount up to about 8 to 9 atomic percent in U.S. Pat. No. 3,684,714 by Carl et al. for the purpose of improving the density and the electromechanical coupling factors. The lanthanum is added in the form of lanthanum oxide. In U.S. Pat. No. 3,549,536 by Lungo et al., a PZT composition consisting essentially of lead zirconate and lead titanate in a mol ratio from about 65:35 to 40:60 and manganese oxide in an amount from about 0.05 to 0.8 weight percent or consisting essentially of lead zirconate and lead titanate in the above mol-ratio range, of manganese oxide and chromium or iron or uranium oxide, and from 0 to 20 atomic percent of at least one alkaline earth selected from the class consisting of barium, calcium, strontium and magnesium, which is added as a carbonate, are disclosed as having an improved mechanical quality factor (Q). The presence of alumina is shown to decrease the density of PZT ceramics in U.S. Pat. No. 3,856,693 by Kim.
Halides have been added to ceramics to increase density and/or mechanical strength. Density of cured PZT ceramics is increased in U.S. Pat. No. 3,021,441 by G. N. Howatt by the substitution of lead fluoride and/or chloride for up to 70 mole percent of lead oxide. In U.S. Pat. No. 3,753,911 by Walker et al. an alkali fluoride or chloride is added to barium titanate to increase density and mechanical strength of the ceramic. In U.S. Pat. No. 3,862,046 by Walker et al., an alkaline earth halide is added in combination with magnesium or lanthanum oxide to achieve the same objective.
Halides have also been included in ceramics to modify properties other than density. In U.S. Pat. No. 2,989,483 by W. S. Miller, lithium or copper chloride is included in a barium titanate ceramic which becomes polarized without an external magnetic field. The inclusion of sodium and/or potassium chloride is added to an aqueous suspension of the oxides of lead, titanium, and zirconium improves the uniformity of the resulting PZT ceramic in U.S. Pat. Nos. 4,152,280 and 1 by Arendt et al.